The prior art is replete with smog abatement processes using ammonia to react with sulphur dioxide, but recovery of ammonium compounds for sale as byproducts increase power plant costs instead of reducing them. Likewise the resulting ammonium-sulphur compounds and particulates in minute amounts produce a plume and smog from power plant chimneys, which is as objectionable as a sulphur oxide smog.
Currently, the least expensive processes being used for power plant pollution abatement provide for scrubbing the flue gas with a limestone slurry. But, locally mined limestone is rarely available, and capital and operating costs for grinding the limestone, and disposing of precipitated gypsum mixed with fly ash, is expensive. Additional costs for electrostatic precipitators or baghouses are the rule rather than the exception. Typically, the amount of limestone or dolomite required to capture all the sulphur from a high sulphur coal is about equal to the weight of ash in the coal, so massive amounts are needed.
Heretofore, power plant recuperators of heat from flue gas have been so subject to corrosion by sulphur acids condensing out from the gas at temperatures of 300.degree. F. or less that they have not been economical, since downtime for repairs is enormously expensive.
In the prior art, flue gas traveling at high velocities has been exposed to scrubber water to remove SO.sub.2, wherein film coefficients slow absorption. The physical chemistry at gas speeds of only a foot a second, where fog particles are involved, is quite different from that when speeds of 10 to 40 feet per second existant in conventional practice. The enormous cost of acid resistant metal vessels made large enough to reduce speeds of gas flow through them has prevented them from even being considered.
It is noted in passing that in my previous U.S. Pat. No. 4,054,246, there is disclosed the novel concept of preheating air needed for combustion of power plant fuel by recuperation of the heat in flue gases, using subterranean pits filled with gravel. Further, in my U.S. Pat. No. 4,173,034, the advantages of heat storage in very large beds of pebbles are disclosed in some detail.
Those skilled in the chemical process arts generally appreciate that regenerative and recuperative heaters have been known for centuries. That this is still an active area of development is evidenced by three recent U.S. patents, U.S. Pat. No. 4,383,573; U.S. Pat. No. 4,349,069; and U.S. Pat. No. 4,361,183, all assigned to Combustion Engineering Inc., which all disclose regenerative heaters, but which are not otherwise pertinent to the present invention.
While the terms "regenerative" and "recuperative" are frequently used in the alternative it is believed that more proper definitions suggest that "regenerative" implies a closed system whereas "recuperative" implies systems where energy may be added (e.g. not a closed system). Insofar as the present invention is of the former type "regenerative" will be used herein.